Aircraft fan engines present problems of avoiding an overspeed condition when the blades are in either a coarse pitch configuration or a fine pitch configuration. Heretofore, flyball governors might be employed for sensing an overspeed condition and rendering a signal to change the blade angles to prevent or counteract the condition.
For instance, a flyball governor might be employed to override a servo valve and effectively shuttle the valve which, in essence, changes the pressures to a hydraulic motor which controls the blade pitch. During normal operation of the aircraft engine, the blade pitch is in a zone sometimes called a "feather" region. This region is between the fine pitch and the coarse pitch condition of the blades for propelling the aircraft forward in normal operation. In other words, the speed of the engine is being controlled in the feather region of the blades between fine and coarse pitch condition. Should the engine overspeed in the feather region, the governor means would take over and effectively cause the blades to be rotated toward coarse pitch condition (i.e., more parallel to the air flow) to reduce the resistance and slow down the engine.
However, an overspeed condition also can occur in what is called the "thrust reverse" region of the blades, as when landing the aircraft. If a conventional governor system as described is employed, the system would be effective to rotate the blades first through their fine pitch condition before proceeding further to the coarse pitch condition. In essence, this has the affect of speeding the engine up before it can be slowed down. This would result in a more serious overspeed condition before the situation actually is rectified.
This invention is directed to providing a new and imroved overspeed protection system which avoids this problem by sensing the overspeed condition when in thrust-reverse mode and driving the blade motor means further into thrust-reverse on actuation by the governor means, thereby avoiding a situation of effecting a more serious overspeed condition.